The present invention relates to a steering control apparatus for a vehicle, and particularly relates to a steering control apparatus for reducing steady-state and transient torque steer resulted from layouts and characteristics of drive shafts of a vehicle.
In general, in a steering apparatus for a vehicle with steered wheels thereof being served as its driving wheels, such a phenomenon that steering effort or steer-holding force shall vary in response to variation of driving force, is called as torque steer, which is desired to be restrained. For example, Japanese Patent Laid-open Publication No. 11-129927, which corresponds to U.S. Pat. No. 6,154,696, discloses a vehicle equipped with an electric power steering device and a torque split arrangement for individually controlling (or, distributing) traction and/or braking force between right and left wheels), to improve its maneuverability and stability. In the U.S. Pat. No. 6,154,696, there is disclosed a steering control system for controlling torque steer in a vehicle equipped with an electric power steering device and a torque split arrangement for individually controlling traction and/or braking force of right and left wheels, which comprises a torque difference input unit for receiving a torque difference signal corresponding to a difference in traction and/or braking force between the right and left wheels, a torque steer canceling steering torque determining unit for producing a torque steer canceling steering torque signal that is required to cancel a steering torque arising from the difference in traction and/or braking force between the right and left wheels, and a drive circuit for supplying a drive current to the electric power steering device according to the torque steer canceling steering torque signal.
Furthermore, according to Japanese Patent Laid-open Publication No. 2005-170116, an apparatus is proposed to solve such a problem in the prior art as described above that the torque steer cancelling control is performed only when the difference in force was caused between the right and left wheels, i.e., difference in rotation was caused between the right and left wheels. That is, there is proposed a steering control apparatus, wherein, instead of the difference in rotation between the right and left wheels, by watching a difference in transferring torque between right and left driving axles, which might cause the torque steer, detecting or estimating engine torque to cancel the torque steer, and obtaining an estimated value of torque steer by a memory circuit which memorizes a relationship of the difference in transferring torque between the right and left driving axles against the engine torque, to cancel the torque steer created by the difference in transferring torque between the right and left driving axles. With respect to the vehicle with steered wheels thereof being served as its driving wheels, according to Japanese Patent Laid-open Publication No. 5-77653, a driving force distribution apparatus is proposed for distributing the driving force to right and left wheels of a vehicle of four-wheel drive system, or the like.
However, it is still difficult to sufficiently reduce a transient torque steer as described later, according to compensation based on the relationship of the difference in transferring torque between the right and left driving axles against the engine torque, as described in Japanese Patent Laid-open Publication No. 2005-170116.
Hereinafter, the cause for creating the torque steer will be analyzed. The torque steer is meant by a phenomenon, wherein according to a front engine front drive vehicle (so called FF vehicle) or four-wheel drive vehicle with the steered wheels thereof being served as its driving wheels, the steering wheel is steered by the steered wheels, when the vehicle is accelerated, i.e., the phenomenon with the steered wheels provided for steering the steering wheel. As for the cause for creating the torque steer, mainly raised are “bent angle of a constant speed universal joint for drive shafts” and “difference in driving force between right and left wheels, when a kingpin offset is provided.”
At the outset, will be explained “torque steer caused by bent angle of a constant speed universal joint for drive shafts”, as (1). With respect to the relationship between the drive shafts and wheels, supposing that the bent angle θ of the constant speed universal joint for the drive shafts is provided, as shown in FIG. 17, if the driving torque transferred by the drive shafts is indicated by “Tdrv”, secondary couple moment (Mz) is created for steering the wheels, according to the following equation (1);Mz=Tdrv·tan(θ/2)  (1)
In FIG. 18, according to the vehicle with the steered wheels thereof being served as its driving wheels, a part of it including its steering apparatus is disclosed to clarify the corresponding relationship between its front view and plan view. That is, in FIG. 18, according to the vehicle with an engine EG and a transmission TR arranged in a transverse direction to the vehicle moving direction, to obtain a space efficiency in an engine compartment, length and arrangement of the drive shafts (maybe called as drive axles) DS1 and DS2 are not provided symmetrically in the transverse direction. Therefore, in the case where the joint bent angles for the drive shafts connected to the driving wheels are different between the right and left wheels WH1 and WH2, a moment (Mz) for steering the wheels, or called as a steered torque, will be caused to provide its difference between the right and left wheels WH1 and WH2, to create the torque steer for steering the steering wheel SW by the steered wheels when the vehicle is accelerated. Thus, the torque steer caused by the bent angle of the constant speed universal joint for the drive shafts is called as the steady-state torque steer.
Next will be explained “torque steer caused by the difference in driving force between the right and left wheels, when the kingpin offset is provided”, as (2). As shown in FIG. 18, the steered wheels WH1 and WH2 are provided with kingpins KP1 and KP2 to be capable of being steered, and the position of the steered center TC, i.e., intersection of the kingpin axis and road surface, does not correspond to the position of force applied point DP of the driving force, so that there exists a distance between those two points, i.e., kingpin offset (KPo), whereas (KPc) in FIG. 18 designates a wheel center kingpin offset. In the case where there exists the kingpin offset (KPo), when the vehicle is accelerated to apply the driving force to the steered wheels WH1 and WH2, the torque for steering the steered wheel, i.e., steered torque, is created, which can be obtained by [driving force]×[kingpin offset]. If the driving force is equal to each other between the right and left wheels WH1 and WH2, the steered torque will be cancelled, so that no torque steer will be caused. If the driving force is different from each other between the right and left wheels WH1 and WH2, however, “torque steer for steering the steering wheel by means of the steered wheel (right and left wheels)” will be caused.
As for the case where the driving force is different from each other between the right and left wheels as described in (2), the following three cases may be considered:    (2-a) “driving force difference between right and left wheels due to characteristics of drive shafts”In the case where there exists a difference in characteristic between the drive shafts DS1 and DS2, a transient (dynamic) difference will be caused in transferring the torque. Even in the case where the drive shafts DS1 and DS2 are made of the same material, and formed with the same cross sectional area, if their lengths are different from each other, torsional rigidity of them shall be different from each other. Therefore, when the vehicle is accelerated rapidly, the driving force applied to the wheel connected with the drive shaft, which is relatively short to provide relatively high torsional rigidity, will be increased rapidly with a slight delay. On the contrary, the driving force applied to the wheel connected with the drive shaft, which is relatively long to provide relatively low torsional rigidity, will be increased gradually. Therefore, the difference in transient driving force will be caused between the right and left wheels, thereby to create the torque steer, which is called as the transient torque steer.    (2-b) “driving force difference between right and left wheels due to a traction control”If braking torque is applied to one wheel according to a traction control, the driving force applied to the other one wheel corresponding to the braking torque will be increased. Particularly, in the case where the traction control is performed on a so-called μ-split road with different coefficients of friction provided between the right and left wheels, the driving force difference between the right and left wheels will be caused largely.    (2-c) “driving force difference between right and left wheels due to a driving force distribution device”In the case where a driving force distribution device is provided between the right and left wheels, the difference in driving force between the right and left wheels will be caused. As for the driving force distribution device, there are known the one controlled electronically, and the one for limiting their differential mechanically, e.g., viscous coupling or the like, as disclosed in the aforementioned Japanese Patent Laid-open Publication No. 5-77653, for example.